A hash table supporting full concurrency of retrievals and
adjustable expected concurrency for updates. This class obeys the
same functional specification as Hashtable, and
includes versions of methods corresponding to each method of
Hashtable. However, even though all operations are
thread-safe, retrieval operations do not entail locking,
and there is not any support for locking the entire table
in a way that prevents all access. This class is fully
interoperable with Hashtable in programs that rely on its
thread safety but not on its synchronization details.

Retrieval operations (including get) generally do not
block, so may overlap with update operations (including
put and remove). Retrievals reflect the results
of the most recently completed update operations holding
upon their onset. For aggregate operations such as putAll
and clear, concurrent retrievals may reflect insertion or
removal of only some entries. Similarly, Iterators and
Enumerations return elements reflecting the state of the hash table
at some point at or since the creation of the iterator/enumeration.
They do not throw ConcurrentModificationException.
However, iterators are designed to be used by only one thread at a time.

The allowed concurrency among update operations is guided by
the optional concurrencyLevel constructor argument
(default 16), which is used as a hint for internal sizing. The
table is internally partitioned to try to permit the indicated
number of concurrent updates without contention. Because placement
in hash tables is essentially random, the actual concurrency will
vary. Ideally, you should choose a value to accommodate as many
threads as will ever concurrently modify the table. Using a
significantly higher value than you need can waste space and time,
and a significantly lower value can lead to thread contention. But
overestimates and underestimates within an order of magnitude do
not usually have much noticeable impact. A value of one is
appropriate when it is known that only one thread will modify and
all others will only read. Also, resizing this or any other kind of
hash table is a relatively slow operation, so, when possible, it is
a good idea to provide estimates of expected table sizes in
constructors.

This class and its views and iterators implement all of the
optional methods of the Map and Iterator
interfaces.

Like Hashtable but unlike HashMap, this class
does not allow null to be used as a key or value.

ConcurrentHashMap

Creates a new map with the same mappings as the given map.
The map is created with a capacity of 1.5 times the number
of mappings in the given map or 16 (whichever is greater),
and a default load factor (0.75) and concurrencyLevel (16).

contains

Legacy method testing if some key maps into the specified value
in this table. This method is identical in functionality to
containsValue(java.lang.Object), and exists solely to ensure
full compatibility with class Hashtable,
which supported this method prior to introduction of the
Java Collections framework.

Parameters:

value - a value to search for

Returns:

true if and only if some key maps to the
value argument in this table as
determined by the equals method;
false otherwise

clear

keySet

Returns a Set view of the keys contained in this map.
The set is backed by the map, so changes to the map are
reflected in the set, and vice-versa. The set supports element
removal, which removes the corresponding mapping from this map,
via the Iterator.remove, Set.remove,
removeAll, retainAll, and clear
operations. It does not support the add or
addAll operations.

The view's iterator is a "weakly consistent" iterator
that will never throw ConcurrentModificationException,
and guarantees to traverse elements as they existed upon
construction of the iterator, and may (but is not guaranteed to)
reflect any modifications subsequent to construction.

values

Returns a Collection view of the values contained in this map.
The collection is backed by the map, so changes to the map are
reflected in the collection, and vice-versa. The collection
supports element removal, which removes the corresponding
mapping from this map, via the Iterator.remove,
Collection.remove, removeAll,
retainAll, and clear operations. It does not
support the add or addAll operations.

The view's iterator is a "weakly consistent" iterator
that will never throw ConcurrentModificationException,
and guarantees to traverse elements as they existed upon
construction of the iterator, and may (but is not guaranteed to)
reflect any modifications subsequent to construction.

entrySet

Returns a Set view of the mappings contained in this map.
The set is backed by the map, so changes to the map are
reflected in the set, and vice-versa. The set supports element
removal, which removes the corresponding mapping from the map,
via the Iterator.remove, Set.remove,
removeAll, retainAll, and clear
operations. It does not support the add or
addAll operations.

The view's iterator is a "weakly consistent" iterator
that will never throw ConcurrentModificationException,
and guarantees to traverse elements as they existed upon
construction of the iterator, and may (but is not guaranteed to)
reflect any modifications subsequent to construction.